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GRK 1640:  Photophysics of Synthetic and Biological Multichromophoric Systems

Subject Area Statistical Physics, Nonlinear Dynamics, Complex Systems, Soft and Fluid Matter, Biological Physics
Term from 2010 to 2019
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 148646377
 
Final Report Year 2020

Final Report Abstract

Most of the colours that we experience in daily life result from molecules that absorb and emit light in the visible spectral range and which are therefore referred to as chromophores or dyes. The colours result from the interaction of the dye or pigment molecules with (visible) light. A multichromophoric system is a group of, say, dye molecules that are interacting with each other, for example within a molecular assembly. Owing to the intermolecular interaction the optical excitations of such assemblies can be shared between many monomers leading to delocalized excitations, so-called excitons. These excitons can migrate along the assembly, which makes molecular aggregates, i.e. multichromophoric systems, highly interesting for transporting (solar) energy that can be used for subsequent charge separation. In other words, such systems are promising candidates for the development of novel solar cells based on organic matter. Indeed in photosynthesis, i.e. the natural counterpart of organic photovoltaics, molecular aggregates organized in protein scaffolds play a key role for the efficiency of this energy conversion process. It turns out that the actual nature of the electronically excited states is determined by a complex interplay of various parameters such as the site energy of the monomers (and the variation thereof), the electronic coupling between the monomers (and the variation thereof), the coupling between the electronic states and vibrations (electron-phonon coupling), and the coupling between the monomers and the local environment. Aim of the RTG was to reveal and control the interplay between these parameters to elucidate the fundamental processes that govern energy transfer, charge transfer, and energy conversion processes in multichromophoric systems of both natural and man-made origin. One class of materials studied were conjugated polymers. The influence of aggregation on the photophysical parameters was studied, and a central finding was that for different polymers the aggregate formation as a function of temperature proceeds in a similar coil-to-globule like phase transition process. Yet, supramolecular chemistry offers the opportunity to control the mutual arrangement of the monomers within molecular aggregates by chemical modifications, e.g., by adding/removing hydrogen-bonding or hydrophilic/-phobic groups. In various approaches this allowed i) to elucidate the influence of steric hindrance on the geometric structure formation and concomitantly its influence on the character of the electronic states, and/or ii) to exploit a specifically synthesized supramolecular building block that features a self-assembly in long nanofibres to form a H-aggregate, and which allowed us to verify partly coherent excitation energy transfer over micrometre distances. For the natural materials we employed photosynthetic pigment-protein complexes that accommodate dye molecules in a well-defined arrangement. Owing to the much more ordered arrangement of the chromophore molecules their electronic excitations correspond to molecular exciton states that are shared by several monomers, i.e. the electronic wavefunctions feature (some degree of) coherence. Our work revealed that (moderate) disorder and coherence are not mutually exclusive but that their interplay can work hand in hand for accomplishing an efficient light harvesting antenna structure.

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